Liquid crystal displays (LCDs) have found wide applications in modern electronic devices such as personal computer screens, liquid crystal televisions, cell phones or Personal Digital Assistants (PDAs) due to their advantageous characteristics of low power consumption, light weight, thin profile, etc.
In general, an LCD controls light transmittance of liquid crystal by an electrical field so as to display images. In terms of the electrical field's driving direction, liquid crystals can be roughly sorted into a horizontal electrical field type and a vertical electrical field type. An LCD of the horizontal electrical field type drives the liquid crystal in an In-Plane Switching (IPS) mode by using a horizontal electrical field formed between a pixel electrode and a common electrode that are provided parallel to each other on a lower substrate. An LCD of the vertical electrical field type drives the liquid crystal in an Twisted Nematic (TN) mode by using the vertical electrical field between a pixel electrode and a common electrode that are provided opposite to each other on a lower and an upper substrates respectively.
For LCDs, there is a simplified matrix type, an active matrix type in which active elements such as TFTs (Thin Film Transistors) are used in pixels, and so on. The driving methods of an LCD of the active matrix type include frame-inversion driving, H line-inversion (row-inversion) driving, V line-inversion (column-inversion) driving, dot-inversion driving, and the like. LCDs of the active matrix type using different driving methods will be described below by way of example with reference to the accompanying drawings.
An LCD using column-inversion driving is schematically described with reference to FIGS. 1 and 2. FIG. 1 is a schematic view of the polarities of pixels of a liquid crystal panel of an LCD when the LCD employs column-inversion driving. As shown in FIG. 1, in the current frame, the polarities of the pixels of odd columns are positive, and the polarities of the pixels of even columns are negative. FIG. 2 is a schematic circuit diagram of part of pixels of the liquid crystal panel as shown in FIG. 1. For the purposes of clarity and ease of illustration, only part of the circuit structure is shown in FIG. 2. As shown in FIG. 2, gate electrodes 20g of TFTs 20 in the pixels in the same row are connected to the same scanning line, the pixels in the same column have the same polarity, source electrodes 20s of TFTs 20 in the pixels in the same column are connected to the same data line, and a drain electrode 20d of TFT 20 in each of the pixels is connected to a corresponding pixel electrode 22. For example, the gate electrodes 20g of TFTs 20 in pixels of the first row are connected to a scanning line G1, the pixels of the first column have positive polarity in the current frame, and the source electrodes 20s of TFTs 20 in the pixels of the first column are connected to a data line D1.
An LCD using dot-inversion driving is schematically described with reference to FIGS. 3 and 4. FIG. 3 is a schematic view of the polarities of pixels of a liquid crystal panel of an LCD when the LCD employs the dot-inversion driving. As shown in FIG. 3, in contrast to column-inversion driving and other driving methods, the polarity of each pixel is different from that of adjacent columns and adjacent rows of the pixels thereof FIG. 4 is a schematic circuit diagram of part of the pixels of the liquid crystal panel as shown in FIG. 3. Also for the purposes of clarity and ease of illustration, only part of the circuit structure is shown in FIG. 4. As shown in FIG. 4, gate electrodes 40g of TFTs 40 in the pixels of odd columns in the two adjacent rows are connected to the same scanning line, gate electrodes 40g of TFTs 40 in the pixels of even columns in the two adjacent rows are connected to another scanning line, source electrodes 40s of TFTs 40 in the pixels in the same column that have the same polarity are connected to the same data line, and a drain electrode 40d of the TFT 40 in each of the pixels is connected to a corresponding pixel electrode 42. For example, in FIG. 4, the gate electrodes 40g of the TFTs 40 in the pixels P11, P13, P21, and P23 are connected to the scanning line G1, the gate electrodes 40g of the TFTs 40 in the pixels P22, P24, P32, and P34 are connected to the scanning line G2, and the source electrodes 40s of the TFTs 40 in the pixels P11 and P31 are connected to the data line D1.
A conventional liquid crystal panel of the active matrix type generally comprises n rows of scanning lines that are parallel to each other, m columns of data lines that are parallel to each other and that are perpendicular to and insulated with the n rows of scanning lines, and a plurality of pixels. Each pixel comprises a TFT, a liquid crystal capacitor CLC, and a storage capacitor Cst. The TFT is located at the intersection of a scanning line and a data line, and functions as a switch element to drive a pixel electrode. The gate electrode of the TFT is connected to a scanning line so as to receive scanning signals transmitted by the scanning line, the source electrode is connected to a data line, and the drain electrode is connected to the pixel electrode. The minimum region surrounded by the scanning lines and the data lines is defined as a pixel region. Each row of pixels includes m pixel electrodes. A liquid crystal capacitor CLC is formed between a pixel electrode and a common electrode of an opposite substrate (also referred to as an opposite electrode). A storage capacitor Cst is formed between a pixel electrode and a common electrode of an array substrate (also referred to as a storage electrode).
A first end of a liquid crystal capacitor (i.e., pixel electrode) is coupled to a data line via the drain electrode and the source electrode of a TFT, and a second end of the liquid crystal capacitor is connected to an opposite substrate to receive a common voltage signal Vcom. A first end of a storage capacitor is connected with the first end of the liquid crystal capacitor, and a second end of the storage capacitor is connected to an array substrate to receive the common voltage signal Vcom. When the liquid crystal panel is scanned, a plurality of scanning signals are generated in the time of a frame, which are applied to the respective scanning lines. When a TFT is turned on by a scanning signal, a data signal voltage is transmitted to the first end of the liquid crystal capacitor and the first end of the storage capacitor through the source electrode and the drain electrode of the TFT, thereby charging the liquid crystal capacitor and the storage capacitor.
The waveforms of the pixel voltage, data signal voltage, common voltage signal and scanning signal of a pixel in a conventional liquid crystal panel (for example, the liquid crystal panels as shown in FIGS. 1-4) when it is driven will be described with reference to FIGS. 5A and 5B. In FIGS. 5A and 5B, Vgh represents the high voltage of a scanning line (also referred to as scanning start-up signal), Vgl represents the low voltage of the scanning line, Vsig1 and Vsig2 represent data signal voltages supplied to a data line, Vpixel represents a voltage by which a pixel electrode is charged (also referred to as pixel voltage), and Vcom represents the common voltage signal that is supported to the common electrode of the pixel.
FIG. 5A is a waveform diagram of the pixel voltage, data signal voltage, common voltage signal and scanning signal of a certain pixel connected on an odd data line as shown in FIGS. 2 and 4. For the purposes of clarity and ease of illustration, only the waveforms regarding to the nth and (n+1)th frames are shown in FIG. 5A. As shown in FIG. 5A, for the nth frame, during applying the scanning start-up signal to a certain scanning line, the high voltage Vgh turns on the TFTs connected on the scanning line, that is, the drain electrodes and source electrodes s of the TFTs feed through. During this period, a data signal voltage Vsig1 that represents the pixel voltage of the nth frame is applied by the data line to the pixel electrodes through the source electrodes and drain electrodes of the TFTs, whereby the pixels connected on the scanning line show a pixel voltage Vpixel and the storage capacitors in these pixels are in a charging state. The pixel voltage Vpixel is maintained by the storage capacitors within the subsequent time of the frame. Theoretically, the pixel voltage Vpixel shown by the pixels connected on the scanning line will maintain unchanged before the scanning start-up signal of the (n+1)th frame is applied to the scanning line. In practice, however, at the moment the scanning signal transits from the high voltage Vgh to a low voltage Vgl, the TFTs cut off, the charging voltages of the liquid crystal capacitors that are maintained by the storage capacitors will drop suddenly due to the capacitance coupling effect, and decreases a little due to the influence of the adjacent parasitic resistances after the scanning signal keeps at the low voltage Vgl.
When the scanning start-up signal of the (n+1)th frame is applied to the scanning line, the polarity of the pixels connected on the scanning line is reversed. Similarly to the nth frame, during applying the scanning start-up signal of the (n+1)th frame to the scanning line, the high voltage Vgh turns on the TFTs connected on the scanning line. Meanwhile, a data signal voltage Vsig1 that represents the pixel voltage of the (n+1)th frame is applied by the data line to the pixel electrodes through the source electrodes and drain electrodes of the TFTs, whereby the pixels connected on the scanning line are updated to show a pixel voltage Vpixel of the (n+1)th frame and the storage capacitors in these pixels are in a charging state. The pixel voltage Vpixel is maintained by the storage capacitors within the subsequent time of the frame. It goes repeats continuously as such.
FIG. 5B is a waveform diagram of the pixel voltage, data signal voltage, common voltage signal and scanning signal of a certain pixel connected on an even data line as shown in FIGS. 2 and 4. For the purposes of clarity and ease of illustration, only the waveforms regarding to the nth and (n+1)th frames are shown in FIG. 5B. The waveform of the pixel voltage of the pixels connected on even data lines is inverted from that of the pixels connected on odd data lines because the polarity of the pixels connected on the even data lines is inverse from that of the pixels connected on the odd data lines, that is, because the pixel voltage of the pixels connected on the even data lines is inverse in a same frame from that of the pixels connected on the odd data lines. Since in this case, FIG. 5B can be clearly understood with reference to the description made in FIG. 5A, the detailed description thereof is omitted herein.
When a conventional liquid crystal panel displays pixel voltages, if there is a difference between the images of two consecutive frames of pictures, then image sticking is readily produced. This is because the liquid crystal material has a slow reaction speed and long reaction time. Moreover, when an object in the pictures moves rapidly, the liquid crystal material cannot track the movement of the object in real time during scanning one picture. In this case, what the liquid crystal material generates is the accumulative reaction of several instances of picture scanning. In order to solve the afterimage problem, a large number of research reports have been proposed with respect to the special characteristics of the liquid crystal material, which focus on the following aspects: (1) intrinsic properties: changing the stickiness of liquid crystal to a low viscosity; (2) increasing the twisting voltage, i.e., over driving: so that the liquid crystal twists and restores more quickly; and (3) inserting a fully black picture (for short, black insertion): inserting a fully black picture after each video picture has been displayed and before a next video picture is displayed.
However, if the way of changing the stickiness of the liquid crystal is used to improve the quality of the dynamically displayed pictures, other parameters and characteristics of the liquid crystal will be changed accordingly, which causes other disadvantageous effects. If the way of over driving is used, the driving voltage needs to be increased or voltage compensation needs to be adopted, which has a high requirement for the driving circuit. If the existing black insertion technology is used, a source driving circuit has to generate alternatively video data and fully black data. That is, the video data and the black insertion data are both generated by the source driving circuit. Since the source driving circuit has to generate black insertion voltages and data driving voltages at different time, the scanning frequency of a gate driving circuit has to be increased, for example doubled, tripled, and so on, whereby the load of the source driver increases considerably and the reaction speed of the source driver has to be improved accordingly.